執行緒池之ThreadPoolExecutor使用
阿新 • • 發佈:2021-01-11
ThreadPoolExecutor提供了四個構造方法:
ThreadPoolExecutor構造方法.png
我們以最後一個構造方法(引數最多的那個),對其引數進行解釋:
public ThreadPoolExecutor(int corePoolSize, // 1
int maximumPoolSize, // 2
long keepAliveTime, // 3
TimeUnit unit, // 4
BlockingQueue<Runnable> workQueue, // 5
ThreadFactory threadFactory, // 6
RejectedExecutionHandler handler ) { //7
if (corePoolSize < 0 ||
maximumPoolSize <= 0 ||
maximumPoolSize < corePoolSize ||
keepAliveTime < 0)
throw new IllegalArgumentException();
if (workQueue == null || threadFactory == null || handler == null)
throw new NullPointerException();
this.corePoolSize = corePoolSize;
this.maximumPoolSize = maximumPoolSize;
this.workQueue = workQueue;
this.keepAliveTime = unit.toNanos(keepAliveTime);
this.threadFactory = threadFactory;
this.handler = handler;
}
序號 | 名稱 | 型別 | 含義 |
---|---|---|---|
1 | corePoolSize | int | 核心執行緒池大小 |
2 | maximumPoolSize | int | 最大執行緒池大小 |
3 | keepAliveTime | long | 執行緒最大空閒時間 |
4 | unit | TimeUnit | 時間單位 |
5 | workQueue | BlockingQueue<Runnable> | 執行緒等待佇列 |
6 | threadFactory | ThreadFactory | 執行緒建立工廠 |
7 | handler | RejectedExecutionHandler | 拒絕策略 |
如果對這些引數作用有疑惑的請看 ThreadPoolExecutor概述
知道了各個引數的作用後,我們開始構造符合我們期待的執行緒池。首先看JDK給我們預定義的幾種執行緒池:
一、預定義執行緒池
- FixedThreadPool
public static ExecutorService newFixedThreadPool(int nThreads) {
return new ThreadPoolExecutor(nThreads, nThreads,
0L, TimeUnit.MILLISECONDS,
new LinkedBlockingQueue<Runnable>());
}
- corePoolSize與maximumPoolSize相等,即其執行緒全為核心執行緒,是一個固定大小的執行緒池,是其優勢;
- keepAliveTime = 0 該引數預設對核心執行緒無效,而FixedThreadPool全部為核心執行緒;
- workQueue 為LinkedBlockingQueue(無界阻塞佇列),佇列最大值為Integer.MAX_VALUE。如果任務提交速度持續大餘任務處理速度,會造成佇列大量阻塞。因為佇列很大,很有可能在拒絕策略前,記憶體溢位。是其劣勢;
- FixedThreadPool的任務執行是無序的;
適用場景:可用於Web服務瞬時削峰,但需注意長時間持續高峰情況造成的佇列阻塞。
- CachedThreadPool
public static ExecutorService newCachedThreadPool() {
return new ThreadPoolExecutor(0, Integer.MAX_VALUE,
60L, TimeUnit.SECONDS,
new SynchronousQueue<Runnable>());
}
- corePoolSize = 0,maximumPoolSize = Integer.MAX_VALUE,即執行緒數量幾乎無限制;
- keepAliveTime = 60s,執行緒空閒60s後自動結束。
- workQueue 為 SynchronousQueue 同步佇列,這個佇列類似於一個接力棒,入隊出隊必須同時傳遞,因為CachedThreadPool執行緒建立無限制,不會有佇列等待,所以使用SynchronousQueue;
適用場景:快速處理大量耗時較短的任務,如Netty的NIO接受請求時,可使用CachedThreadPool。
- SingleThreadExecutor
public static ExecutorService newSingleThreadExecutor() {
return new FinalizableDelegatedExecutorService
(new ThreadPoolExecutor(1, 1,
0L, TimeUnit.MILLISECONDS,
new LinkedBlockingQueue<Runnable>()));
}
咋一瞅,不就是newFixedThreadPool(1)嗎?定眼一看,這裡多了一層FinalizableDelegatedExecutorService包裝,這一層有什麼用呢,寫個dome來解釋一下:
public static void main(String[] args) {
ExecutorService fixedExecutorService = Executors.newFixedThreadPool(1);
ThreadPoolExecutor threadPoolExecutor = (ThreadPoolExecutor) fixedExecutorService;
System.out.println(threadPoolExecutor.getMaximumPoolSize());
threadPoolExecutor.setCorePoolSize(8);
ExecutorService singleExecutorService = Executors.newSingleThreadExecutor();
// 執行時異常 java.lang.ClassCastException
// ThreadPoolExecutor threadPoolExecutor2 = (ThreadPoolExecutor) singleExecutorService;
}
對比可以看出,FixedThreadPool可以向下轉型為ThreadPoolExecutor,並對其執行緒池進行配置,而SingleThreadExecutor被包裝後,無法成功向下轉型。因此,SingleThreadExecutor被定以後,無法修改,做到了真正的Single。
- ScheduledThreadPool
public static ScheduledExecutorService newScheduledThreadPool(int corePoolSize) {
return new ScheduledThreadPoolExecutor(corePoolSize);
}
newScheduledThreadPool呼叫的是ScheduledThreadPoolExecutor的構造方法,而ScheduledThreadPoolExecutor繼承了ThreadPoolExecutor,構造是還是呼叫了其父類的構造方法。
public ScheduledThreadPoolExecutor(int corePoolSize) {
super(corePoolSize, Integer.MAX_VALUE, 0, NANOSECONDS,
new DelayedWorkQueue());
}
對於ScheduledThreadPool本文不做描述,其特性請關注後續篇章。
二、自定義執行緒池
以下是自定義執行緒池,使用了有界佇列,自定義ThreadFactory和拒絕策略的demo:
public class ThreadTest {
public static void main(String[] args) throws InterruptedException, IOException {
int corePoolSize = 2;
int maximumPoolSize = 4;
long keepAliveTime = 10;
TimeUnit unit = TimeUnit.SECONDS;
BlockingQueue<Runnable> workQueue = new ArrayBlockingQueue<>(2);
ThreadFactory threadFactory = new NameTreadFactory();
RejectedExecutionHandler handler = new MyIgnorePolicy();
ThreadPoolExecutor executor = new ThreadPoolExecutor(corePoolSize, maximumPoolSize, keepAliveTime, unit,
workQueue, threadFactory, handler);
executor.prestartAllCoreThreads(); // 預啟動所有核心執行緒
for (int i = 1; i <= 10; i++) {
MyTask task = new MyTask(String.valueOf(i));
executor.execute(task);
}
System.in.read(); //阻塞主執行緒
}
static class NameTreadFactory implements ThreadFactory {
private final AtomicInteger mThreadNum = new AtomicInteger(1);
@Override
public Thread newThread(Runnable r) {
Thread t = new Thread(r, "my-thread-" + mThreadNum.getAndIncrement());
System.out.println(t.getName() + " has been created");
return t;
}
}
public static class MyIgnorePolicy implements RejectedExecutionHandler {
public void rejectedExecution(Runnable r, ThreadPoolExecutor e) {
doLog(r, e);
}
private void doLog(Runnable r, ThreadPoolExecutor e) {
// 可做日誌記錄等
System.err.println( r.toString() + " rejected");
// System.out.println("completedTaskCount: " + e.getCompletedTaskCount());
}
}
static class MyTask implements Runnable {
private String name;
public MyTask(String name) {
this.name = name;
}